Frequency control apparatus for an atomic beam tube



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FREQUENCY CONTROL APPARATUS FOR AN ATOMIC BEAM TUBE Filed Jan. 25, 19615 Sheets-Sheet 2 6 Hens 5 l5. 0 O 75 x 22.5 30.ms

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o l l l l l O 7.5 l5. 22.5 30 ms INVENTORS EUGENE F GRANT LAWRENCE FFENTON ATTORNEYS FREQUENCY CONTROL APPARATUS FOR AN ATOMIC BEAM TUBEFiled Jan. 25, 1961 April 30, 1963 E. F. GRANT ETAL 3 Sheets-Sheet 3FIG. 5

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INVENTORS EUGENE F. GRANT LAWRENCE F. FENTON ATTO RNEYS 3,088,078Patented Apr. 30, 1963 3,088,078 FREQUENCY CONTROL APPARATUS FOR ANATOMIC BEAM TUBE Eugene F. Grant, Marblehead Neck, and Lawrence F.Fenton, Maiden, Mass, assiguors to National Company, Inc., Melrose,Mass., a corporation of Massachusetts Filed Jan. 25, 1961, Ser. No.84,880 Claims. (Cl. 331-3) This invention relates to automatic frequencycontrols for frequency standards and more particularly it is concernedwith apparatus for automatically tuning an electronic flywheeloscillator to the resonance frequency of an atomic beam tube or thelike.

Frequency standards incorporating atomic beam tubes conventionally makeuse of a crystal controlled oscillator to provide a useful output signalof standard frequency. To regulate the frequency of the oscillator, aservo system is generally provided. By means of this servo system, thesignal response of the beam tube is used to control the frequency of theoscillator wln'ch in turn is used in synthesizing the beam tube inputsignal.

As is well known to those skilled in the art, the frequency responsewhich the beam tube exhibits has two components, a pedestal, and aseries of symmetrical fluctuations superimposed thereon. The pedestal isoften referred to as the Rabi curve and the superimposed component isknown as the Ramsey pattern. As the central Ramsey peak is normallylocated at the midpoint of the pedestal, it follows that the overallresponse curve is substantially symmetrical about this peak whichcorresponds to the true resonance frequency of the beam tube.

A problem that arises in the design of a servo system to maintain theinput frequency to the beam tube at resonance, that is locked on thecentrally located peak, is the ambiguity introduced by the other maximaor side peaks in the Ramsey pattern. Especially is the problem acutewhen the frequency standard is first placed in operation and the beamtube input signal is displaced substantially from the central peak, asis normally the case. It also can happen after the standard is placed inoperation, that the beam tube input signal frequency shifts to one ofthe side peaks as a consequence ofsome transient phenomenon, and locksonto this peak In patent application No. 59,925 filed-October 3, 19 60,in the name of Arthur Orenberg, there is disclosed an automaticfrequency control system for anatomic frequency standard which offers asolution to this problem based on the use of two modes of servo control,a coarse and a fine control. For coarse control of the oscillatorfrequency, the signal applied to the beam tube is frequency modulatedwith a large frequency deviation encompassing essentially the entirepedestal or Rabi curve. This produces a fundamental component in theoutput of the beam tube at the frequency of the modulating signal, whichapproaches zero as the center operating frequency approaches resonance.This component is used to control the operation of a servo motor whichrough tunes the oscillator frequency. There is also produced in theoutput of the beam tube a second harmonic component which is a maximumat the true resonance frequency. When this second harmonic componentreaches a value slightly less than this maximum signifying that thecenter operating frequency of the beam tube is very nearly the same asthe true resonance frequency, a switching circuit initiates the finemode of oscillator frequency control. For fine control, the signalapplied to the beam tube is frequency modulated with a small deviationencompassing only the Ramsey pattern or a fraction thereof. Thisproduces :a more sharply defined fundamental component in terms offrequency dependence which is adapted to control the operation of theservo motor and so also the oscillator frequency more precisely.

'The main problem encountered with this system is that should theoscillator frequency become locked on a side peak, the system will notinitiate the search for the central peak, unless additional means areprovided for the purpose. Another disadvantage of this system is itsdependence upon an absolute level of second harmonic signal since theamplitude of signal is subject to change for various reasons independentof frequency. For example, operating temperature and ageing have anappreciable effect upon the sensitivity of the beam tube and of theelectronic circuitry, and variations in sensitivity of course arereflected in the amplitude of the second harmonic as well as otherfrequency components of the beam tube output signal. Then too, with thisscheme, an appreciable time delay is involved in the search for the trueresonance frequency before the fine mode of control can be establishedand a useful timing signal obtained from the frequency standard.

Accordingly, the object of the present invention is to provide improvedapparatus for placing an atomic frequency standard in operation at thetrue resonance frequency of the beam tube incorporated in the standard,and for maintaining operation at this frequency.

Another object is to provide a more reliable system of theabove-mentioned character.

Yet another object is to provide a faster acting mode of control of thefrequency of the standard which is adapted to place the standard inoperation more rapidly and to afford closer supervision of itsfrequency.

A further object is to provide frequency control apparatus which doesnot add appreciably to the complexity of the frequency standard.

In brief, the present invention contemplates periodic frequency shiftingof the beam tube input signal between two closely spaced frequencies andtwo relatively widely spaced frequencies. This is accomplished by meansof sine wave or square wave frequency modulation of the beam tube inputfrequency between the desired narrow and wide frequency limits. Thesquare wave FM is done by synthesizing the limit frequencies with singlesideband techniques or by triangular phase modulation of the centerfrequency. In either case, the invention features the use of samplingcircuits or gates which are synchronized with the frequency shifts ofthe beam tube input signal and which provide separate indications of theresponse of the beam tube to small and large frequency shifts. Theseindications in the form of direct current signals are operated uponseparately in two distinct amplifier channels thereby to relate theeffect of large frequency displacements from resonance, which arereflections of the Rabi curve, to the effect of small frequencydisplacements, which are reflections of the Ramsey pattern. Thereafter,the signals are combined in a summing network so as to provide a singlecomposite signal which is adapted to drive the oscillator frequency tothe resonance peak in a continuous fashion irrespective of the magnitudeof any displacement error that is present initially. As in'priorapparatus of a related character, the medium for controlling the crystaloscillator frequency is a servo system.

The novel features of the invention together with further objects andadvantages will become apparent from the following detailed descriptionand the drawings to which it refers. In the drawings:

FIG. 1 is a block diagram of the automatic frequency control apparatusaccording to the present invention;

' FIG. 2 is a graphical representation of the direct voltage output fromthe beam tube as a function of frequency when the signal applied to thebeam tube is unmodulated;

FIG. 3 is a graphical representation of the frequency modulations of thesignal applied to the beam tube;

FIG. 4 is a graphical representation of sample voltages derived from theoutput of the beam tube under various on and off-resonance conditions;

FIG. 5 is a graphical representation of the signals found at certainpoints in the apparatus as a function of frequency displacement errors;and

FIG. 6 is a diagram of a synchronous detector for the sample voltages.

With reference now to the drawings and more particularly to FIG. 1, itwill be observed that the numeral 11 designates the atomic beam tubeemployed in the frequency standard and numeral 12 refers to a frequencysynthesizer and modulator. The function of frequency synthesizer andmodulator 12 is to provide, by means of various frequency multiplying,dividing, and mixing operations, two output signals bearing a fixedfrequency relation to the signal generated by a flywheel oscillator 13,preferably a crystal oscillator. One of these output signals has auseful output frequency for standard reference purposes, such as onemegacycle per second. The other output signal has the capability ofbeing matched in frequency to the atomic resonance frequency of the beamtube to which it is applied. For an atomic beam tube which makes use ofthe lowest atomic resonance of cesium, this frequency is 9192.6318megacycles per sec. An ancillary function of the unit is to apply anglemodulation to the beam tube input signal, that is either phase orfrequency modulation which yields a square waveform to be described indetail hereinafter.

The output signal from the beam tube includes a direct current (DC)component together with an alternating current (A.C.) component whosefrequency is determined by the modulating frequency. Preferably, arelatively low modulating frequency such as twenty cycles per second isused so as to aflord suflicient time for transient phenomena to subsidein the beam tube subsequent to the abrupt changes in frequencyoccasioned by the modulating signal. The output signal from the beamtube is applied to an A.C. amplifier 14 which serves two distinct signalchannels hereinafter referred to as a Ramsey channel and a pedestalchannel. The pedestal channel is seen to include a clamp device 16 whichrefers a selected portion of the beam tube output signal waveform toground. The basic element of the clamp is a switching circuit responsiveto timing pulses transmitted by way of a line 17 from a timing pulse andpattern generator 18. Also included in the pedestal channel is a biasingnetwork and an A.C. amplifier 19. The biasing network in its most basicform includes direct voltage sources 21, 21, and diodes 22, 22. Asshown, the diodes are disposed in parallel circuits with theirpolarities opposite to one another, and the respective sources orbatteries 21 and 21 are in series with the diodes to reverse bias them.Finally there is provided in the pedestal channel a unit 23 whichperforms the function of a synchronous detector under control of timingpulses on lines 24 and 25 from the timing generator. A synchronousdetector unit 26 of like nature is disposed in the Ramsey channel, thisunit being served by timing pulses on lines 27 and 28. Both units 23 and26 will be described in more detail hereinafter in connection with FIG.6.

DC. signals derived by detector units 23 and 26 are combined in asumming circuit 27 and, as amplified in a DC. amplifier 28, are used tocontrol the operation of a servo motor 29. Motor 29 is mechanicallycoupled to a tuning element 13' incorporated in the crystal oscillator13. By this means, fine adjustments are made in the crystal oscillatorfrequency.

The form of synchronous detector circuit contemplated for detector 23 isshown schematically in FIG. 6. From FIG. 6 it -will be observed that thecircuit includes two switches 31 and 32 responsive to pulse signals onlines 24 and 25, respectively. Connected through a resistor 33 and acapacitor 34 to one terminal each of the switches is the ungrounded linefrom amplifier 19 in the pedestal channel. When closed, switch 31effectively grounds the capacitor 34. When switch 32 is closed, itserves to connect capacitor 34 to an integrating circuit including aresistor 36 and a capacitor .37. The output of the detector unit isderived from the capacitor 37, one side of which is also grounded.Various conventional electronic switching circuits incorporating eithertubes or transistors can be used to implement the switches 31 and 32. Inthe Ramsey channel the detector 26 has the same configuration as unit23, the resistor 33 of the Ramsey channel detector being connecteddirectly to the output circuit of common A.C. amplifier 14.

Broadly speaking, the function of the synchronous detector in thepedestal channel is to derive a direct current signal which representsthe variation in the beam tube output signal when the frequency of thebeam tube input signal is shifted between the two more widely separatedlimits defined by the modulating signal. This is the frequency shiftwhich is produced under wideband modulation conditions with a frequencyswing approximately equal to the width of the pedestal, as shown in FIG.3 between +6 and 6 kilocycles per sec. (kc.). Similarly, the function ofsynchronous detector 26 in the Ramsey channel is to derive a directcurrent signal which represents the variation in the beam tube outputsignal when the frequency of the beam tube input signal shifts betweenthe two more closely spaced limits, between +250 and -250 cycles persec. in FIG. 3. This is the frequency shift produced under narrow bandmodulation conditions with a frequency swing approximately equal to thewidth of the Ramsey pattern. The mentioned numbers for the pedestal andRamsey width are typical for certain beam tube constructions, but can bedifferent for other constructions. A more precise understanding of themanner in which the synchronous detectors operate will be gained fromFIG. 1 where the frequency modulation waveform is shown opposite theline 41 on a smaller scale than that of FIG. 3. Also, the control pulsesfor the synchronous detectors are shown adjacent the lines 24, 25, 27,28 over which they are transmitted, the spacing of the pulses withrespect to one another and with respect to the frequency modulationwaveform being representative of their actual time relation. Suchsignals can be generated through the use of conventional pulse circuittechniques incorporating multivibrator circuitry, for example.

From FIG. 1, it will be observed that the control pulses on lines 24 and25, respectively, occur during terminal portions of the respectivepositive and negative going pulses representing the wideband frequencyshifts of the beam tube input signal. During the portion of the positivegoing pulses contemporaneous with a pulse on line 24, switch 31 ofdetector unit 23 closes so that capacitor 34 becomes charged to thevalue of the beam tube output signal. In the case of a frequencydisplacement error of approximately 750 c.p.s. corresponding to point Din FIG. 2, the value to which capacitor 34 is charged at this time is3.5 volts as defined by the 3.75 to 7.5 millisecond (ms.) portion of thecurve D in FIG. 4. Upon termination of the pulse on the line 24, switch31 then opens. Subsequently, during the portion of the negative goingmodulation pulse contemporaneous with a control pulse on the line 25(from 11.25 to 15 ms. in FIG. 4), switch 32 closes while the switch 31remains open. If the value of the beam tube output signal is differentthan it was, as it is in the assumed case of a frequency offset of 750c.p.s., current flows through the switch to ground charging thecapacitor 37. Conversely, if it be assumed that no frequencydisplacement error exists which is the case represented by curve A inFIG. 4, then no charging current flows through the capacitor and nodirect current output signal is produced. In other words, curves A, B,D' in FIG. 4 indicate the changes in the levels of the negative one.

C in FIG. 2..

output signal from the beam tube under various conditions of offsetdefined by the corresponding points A, B, D, in FIG. 2. Prefenably, theduration of the timing pulses should be at least half that of themodulation pulses in order to enhance the signal to noise ratio of thedetected signals. However, the control pulse should begin only after thetransient in the beam tube signal, which is initiated by the suddenchange of frequency, has subsided to a negligible level.

By now, it should be apparent that during the intervals of from 18.75 to22.5 and 26.25 to 30 milliseconds when the small amplitude frequencyshifts occur, the synchronous detector unit 26- in the Ramsey channel isenabled to produce a D.C. output signal representing the variations inthe amplitude of the beam tube output signal. At these times, it will beobserved from FIG. 4 that the change in the beam tube output signal ismuch greater owing to the fact that the beam tube output changes morewith a narrowb and frequency swing than with a wideband frequency swingin the vicinity of its true resonance frequency. Preferably, the timeconstant defined by resistor 36 and capacitor 67 should be largecompared with the time interval defined by one complete modulationcycle. The reason is that integration of the beam tube output signalduring a number of sample periods can be effected in this way, whichenhances the signal to noise ratio of the detected signal. Also the timeconstant defined by resistor 33 and capacitor '34 should be suflicientlylarge to cause (good averaging of noise fluctuations which occur duringa single sample period corresponding to one modulation pulse width.

In FIG. 5, curve R represents the amplitude of the D.C.

signal derived by the synchronous detector 26 in the Ramsey channel as afunction of frequency displacement error or offset. As is apparent, onlydisplacement errors of one sense namely negative displacements in thedirection of decreasing frequency have been illustrated for the sake ofsimplicity in the drawing. Thus, at the origin of the curve,corresponding to no offset, no detector signal is produced since thebeam tube output signal is the same for a positive shift of 250 cyclesper sec. as it is for a For very small displacement enrors it will beobserved that the detector signal increases with increasing frequencydisplacement. For example, it will be seen from FIG. 5 that a frequencyswing about point B in FIG. 2 in the amount of :250 cycles per sec. thenarrow band PM case with which the Ramsey channel is concerned, resultsin a detected signal having 'an amplitude of alppnoximately three volts.When the offset exceeds approximately 250 cycles per sec, however, theeffect of the frequency swing begins to decreasebecause the negativeexcursion of the beam tube signal frequency begins to embrace theresponse minimum shown at point In fact, when the offset becomesapproximately equal to 500 cycles per sec. corresponding to the locationof the minimum, then the detector signal falls to zero owing to thesymmetry of the beam tube frequency response for small frequency shiftsabove and below this point. Thereafter, the detector signal increases inthe negative direction to a maximum at approximately 750 cycles per sec.corresponding to point D in FIG. 2 and once again disappears atapproximately 1000 cycles per see. where another maximum or side peak inthe beam tube response occurs, as shown at E in FIG. 2. Finally, it willbe observed that the signal detected in the Ramsey channel exhibits amaximum at F or approximately 1250 cycles per sec. where the slope ofthe frequency response of the beam tube undergoes a transition fromconcave to convex approximately midway between points E and G in FIG. 2;and another maximum at H where the final point of inflection occurs inthe beam tube response curve of FIG. 2.

Curve P in FIG. 5 represents the amplitude of the signal derived bydetector 23 in the pedestal channel as a function of frequency offsetfrom the true resonance frequency. In the region between the origin ofthe curve and approximately 200 cycles per see, no output signal isdeveloped by the detector because the beam tube out put signal is ofinsufiicient amplitude to overcome the effect of the biasing circuit inthis channel. By this means, full control of the frequency correctionprocess is left to the signal derived in the Ramsey channel whenever thefrequency of the input signal to the beam tube is within the rangeoccupied by the central resonance peak from approximately +200 to --200cycles per sec. Otherwise, any asymmetry of the Ramsey pattern withrespect to the pedestal would have an influence upon the frequency atwhich the system stabilized. Thereafter, with offsets of from 200 toapproximately 500 cycles per see, the pedestal detector signal exhibitsa linear increase with increasing frequency displacement errors,manifesting the asymmetrical location of points on the pedestal skirtscorresponding to frequencies six kilocycles per sec. above and below anoffset center frequency in this region. The level of the pedestaldetector signal at the limit of this region, namely 500 cycles per sec.below the true resonance frequency, is approximately 8 volts, a levelgreater than that produced in the Ramsey channel. This is due to thegain of amplifier 1'9 whereby control is given to the pedestal signalshould the frequency of the beam input signal drift off the centralresonance peak. Offsets of more than 500 cycles per sec. do not cause anappreciable change in the amplitude of the pedestal signal as amplifier19 is adapted to saturate when this level is reached.

The combined signal resulting from the summation of both detectorsignals in the Ramsey and pedestal channels is represented by curve S inFIG. 5. This curve is seen to have the same general shape as curve Rexceptthat it is displaced upwardly with respect thereto because of thepositive level which curve P adds. A primary attribute of curve S isthat it has an appreciable magnitude of fixed sense throughout the rangeof frequency offsets illustrated, namely from zero to approximatelyeight kilocycles per sec. In consequence, the combined signal is adaptedto drive the servo motor 29 and so also the frequency of oscillator 13quickly into the region where the displacement error is less thanapproximately 200 cycles per sec. Also, since the slope of the curve isrelatively linear from zero to 200 cycles per sec. the signal afiords anoptimum mode of servo control whereby a null balance can be obtained atzero displacement errors corresponding to the true resonance frequencyof the beam tube. For frequencies above resonance, that is positivefrequency displacement errors or offsets, the same curves P, R, S applyexcept that a polarity reversal occurs. The polarity of the signaldetermines the direction of rotation of the servo motor whereby acorrection of the proper sense is initiated to eliminate a displacementerror.

Although the invention has been described in connection with a singlepreferred embodiment, it will be apparent to those skilled in the artthat various modifications and alternatives within the spirit and scopeof the invention are possible. For example, instead of the mechanicallytuned capacitor a mechanically tuned inductor can be employed to controlthe oscillator frequency. In fact, the D.C. voltage can be useddirectly, without a servo motor, to change the capacity of a voltagevariable capacitor (varicap), or the D.C. current can be used to changethe inductance of a saturable inductor. Furthermore, several narrow bandsweep cycles can be employed for each wide band cycle or vice versa. Itis also contemplated that triangular phase modulation of the beam tubeinput signal can be used to obtain the desired frequency modulationwaveform. Accordingly the invention should not be deemed to be limitedto the details of what has been described herein by way of illustration,but rather it should be deemed to be limited only by the scope of theappended claims.

What is claimed is:

1. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means for periodically shifting thefrequency of said beam tube input signal between two closely-spacedvalues and two relatively more widely spaced values, means for derivinga first direct current signal as a function of the amplitude variationof the beam tube output signal when the frequency of the beam tube inputsignal is shifted between said closely-spaced values, means for derivinga second direct current signal as a function of the amplitude variationof the beam tube output signal when the frequency of the beam tube inputsignal is shifted between said widely-spaced values, means to produce acomposite signal as a function of the sum of said first and second D.C.signals, and means to control the center frequency of the beam tubeinput signal in response to said composite signal.

2. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means to produce square-wave modulationof the frequency of said beam tube input signal at alternating high andlow levels, means for deriving a first direct current signal as afunction of the beam tube output signal during a cycle of low levelmodulation of the frequency of the beam tube input signal, means forderiving a second direct current signal as a function of the beam tubeoutput signal during a cycle of high level modulation of the frequencyof the beam tube input signal, means to produce a composite signal as afunction of the sum of said first and second direct current signals, andmeans to control the center frequency of the beam tube input signal inresponse to said composite signal.

3. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means to produce square wave frequencymodulation at alternating high and low levels by synthesizing andswitching the frequency of said beam tube input signal, means forderiving a first direct current signal as a function of the beam tubeoutput signal during a cycle of low level modulation of the frequency ofthe beam tube input signal, means for deriving a second direct currentsignal as a function of the beam tube output signal during a cycle ofhigh level modulation of the frequency of the beam tube input signal,means to produce a composite signal as a function of the sum of saidfirst and second direct current signals, and means to control the centerfrequency of the beam tube signal in response to said composite signal.

4. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, said first named means including acrystal oscillator with a mechanically tunable element for varying thefrequency of the oscillator, means to produce square-wave modulation ofthe frequency of said beam tube input signal at alternating high and lowlevels, means for deriving a first direct current signal as a functionof the beam tube output signal during a cycle of low level modulation ofthe frequency of the beam tube input signal, means for deriving a seconddirect current signal as a function of the beam tube output signalduring a cycle of the high level modulation of the frequency of the beamtube input signal, means to produce a composite signal as a function ofthe sum of said first and second direct current signals, a servo motorcoupled to the mechanically tunable element and means to control theoperation of the servo motor in response to said composite signal.

5. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means to produce square-wave modulationof the frequency of said beam tube input signal at alternating high andlow levels, a pair of signal transmission channels for processing of theoutput signal from said beam tube,

means to produce an increased amount of amplification of the outputsignal transmitted by the second of said channels relative to thattransmitted by the first channel, a first synchronous detector disposedin said first channel for deriving a first direct current signal as afunction of the beam tube output signal during cycles of low levelmodulation of the frequency of the beam tube input signal, a secondsynchronous detector disposed in said second channel for deriving asecond direct current signal as a function of the beam tube outputsignal during cycles of high level modulation of the frequency of thebeam tube input signal, means to produce a composite signal as afunction of the sum of said first and second direct current signals, andmeans to control the center frequency of the beam tube input signal inresponse to said composite signal.

6. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means :to produce square-wave modulationof the frequency of said beam tube input signal at alternating high andlow levels, a pair of signal transmission channels for processing of theoutput signal from said beam tube, an amplifier disposed in said secondchannel to increase the amplitude of the signal transmitted therebyrelative to that transmitted by said first channel, a biasing networkdisposed in said second channel to prevent the transmission of signalswhose amplitude is less than a predetermined minimum, a firstsynchronous detector disposed in said first channel for deriving a firstdirect current signal as a function of the beam tube output signalduring cycles of low level modulation of the frequency of the beam tubeinput signal, a second synchronous detector disposed in said secondchannel for deriving a second direct current signal as a function of thebeam tube output signal during cycles of high level modulation of thefrequency of the beam tube input signal, means to produce a compositesignal as a function of the sum of said first and second direct curentsignals, and means to control the center frequency of the beam tubeinput signal in response to said composite signal.

7. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, means to produce square-wave modulationof the frequency of said beam tube input signal at alternating high andlow levels, a pair of signal transmission channels for processing of theoutput signal from the beam tube, a first synchronous detector disposedin said first channel for deriving a first direct current signal as afunction of the beam tube output signal during cycles of low levelmodulation of the frequency of the beam. tube input signal, saidsynchronous detector including means to sample and store the beam tubeoutput signal voltage during the first half of the low level modulationcycles and means to sample and compare the beam tube output signalvoltage during the second half of the low level modulation cycles withthat having been stored from the first half of the corresponding cycles,a second synchronous detector disposed in said second channel forderiving a second direct current signal as a function of the beam tubeoutput signal during cycles of high level modulation of the frequency ofthe beam tube input signal, said second synchronous detector includingmeans to sample and store the beam tube output signal voltage during thefirst half of the high level modulation cycles and means to sample andcompare the beam tube output signal voltage during the second half ofthe high level modulation cycles with that having been stored from thecorresponding first half cycles, a summing circuit to produce acomposite signal representative of the sum of said first and seconddirect current signals, and means to control the center frequency of thebeam tube input signal in response to said composite signal.

8. Apparatus as claimed in claim 7 wherein said means 9 to sample andcompare includes an integrating circuit for averaging of the directcurrent signals detected during a plurality of modulation cycles.

9. Apparatus as claimed in claim 7 wherein said first synchnonousdetector includes means to clamp to zero level the output signal voltagein one half of the low level modulation cycle and said secondsynchronous detector includes means to clamp to zero level the outputsignal voltage in one half of the high level modulation cycle.

10. Frequency control apparatus for use with an atomic beam tube, saidapparatus comprising means for producing a radio frequency signal andapplying it to said beam tube, said first-named means including acrystal oscillator with a tunable element for varying the frequency ofthe oscillator, means to produce square-wave modulation of the frequencyof said beam tube input signal at alternating high and low levels, apair of signal transmission channels for processing of the output signalfrom the beam tube, tan amplifier disposed in said second channel toincrease the amplitude of the signals transmitted by said second channelduring the high level modulation cycles relative to the amplitude of thesignals transmitted by said first channel during the iow levelmodulation cycles, a biasing network disposed in said second channel toprevent the transmission of signals whose amplitude is less than apredetermined minimum, a synchronous detector disposed in said firstchannel for deriving a first direct current signal as a function of thebeam tube output signal during cycles of low level modulation of thefrequency of the beam tube input signal, said first synchronous detectorincluding means to sample and store the beam tube output signal voltageduring the first half of the low level modulation cycles and means tosample and compare the signal voltage during the second half of the lowlevel modulation cycles with that having been stored from thecorresponding first half cycles, a second synchronous detector disposedin said second channel for deriving a second direct current signal as afunction of the beam tube output signal during cycles of high levelmodulation of the frequency of the beam tube input signal, said secondsynchronous detector including means to sample and store the outputsignal voltage during the first half of the high level modulation cyclesand means to sample and compare the signal voltage during the secondhalf of the high level modulation cycles with that having been storedfrom the corresponding first half cycles, a summing circuit to produce acomposite signal representative of the sum of said first and seconddirect current signals, a servo system coupled to the tunable element,and means to control the operation of the servo system in response tosaid composite signal.

No references cited.

1. FREQUENCY CONTROL APPARATUS FOR USE WITH AN ATOMIC BEAM TUBE, SAIDAPPARATUS COMPRISING MEANS FOR PRODUCING A RADIO FREQUENCY SIGNAL ANDAPPLYING IT TO SAID BEAM TUBE, MEANS FOR PERIODICALLY SHIFTING THEFREQUENCY OF SAID BEAM TUBE INPUT SIGNAL BETWEEN TWO CLOSELY-SPACEDVALUES AND TWO RELATIVELY MORE WIDELY SPACED VALUES, MEANS FOR DERIVINGA FIRST DIRECT CURRENT SIGNAL AS A FUNCTION OF THE AMPLITUDE VARIATIONOF THE BEAM TUBE OUTPUT SIGNAL WHEN THE FREQUENCY OF THE BEAM TUBE INPUTSIGNAL IS SHIFTED BETWEEN SAID CLOSELY-SPACED VALUES, MEANS FOR DERIVINGA SECOND DIRECT CURRENT SIGNAL AS A FUNCTION OF THE AMPLITUDE VARIATIONOF THE BEAM TUBE OUTPUT SIGNAL WHEN THE FREQUENCY OF THE BEAM TUBE INPUTSIGNAL IS SHIFTED BETWEEN SAID WIDELY-SPACED VALUES, MEANS TO PRODUCE ACOMPOSITE SIGNAL AS A FUNCTION OF THE SUM OF SAID FIRST AND SECOND D.C.SIGNALS, AND MEANS TO CONTROL THE CENTER FREQUENCY OF THE BEAM TUBEINPUT SIGNAL IN RESPONSE TO SAID COMPOSITE SIGNAL.